A frequency multiplier is an electronic circuit that takes in a periodic input signal with an input frequency f and multiplies the input frequency f by a factor N to generate an output signal with an output frequency N*f. The output frequency N*f is a harmonic of the input frequency f, where N is an integer two or larger. A block diagram of a known frequency multiplier is illustrated in FIG. 1A. The frequency multiplier 111 in FIG. 1A generates the output frequency N*f, but will inevitably also generate unwanted harmonics Mi*f, where Mi represents integers one or larger but not equal to N. In other words, the output signal from the frequency multiplier 111 will contain signal components at all harmonics of the input frequency f, but only the harmonic at the output frequency N*f is desired. A filter 112 or combination of multiple such filters may be used to suppress the signal components at the unwanted harmonics Mi*f of the output of the frequency multiplier 111, resulting in the output signal with the output frequency N*f. Signal components at the unwanted harmonics Mi*f may still be present despite the filtering but the signal components at the unwanted harmonics Mi*f are low enough in amplitude that the unwanted harmonics Mi*f can be tolerated.
Demand for multi-channel instruments has increased. Multi-channel instrumentation is becoming more common in diverse fields such as wireless communications, quantum computing, aerospace and defense. Multi-channel instruments may work in multiple clock domains and utilize clocks that operate at multiple different frequencies. In multi-channel instruments, one clock frequency may be derived from another clock frequency using a frequency multiplier, a frequency divider, or a frequency converter. Waves at related frequencies may be expected to have a consistent phase relationship. Two waves at the same frequency may be offset from one another in time but will still have a strong relationship. For example, two offset waves at the same frequency will have periodic peaks, lows and crossings of zero separated by the same amount of time, even if the peaks and lows of the two offset waves are at different amplitudes. Two waves with different frequencies will not have the strong relationship of two waves at the same frequency, but two waves of different frequencies may still have a phase relationship of some form. For example, a second wave with a frequency that is an integer multiple of a first wave would be expected to have one or more reoccurring characteristics that coincide with one or more reoccurring characteristics of the first wave, just not at the same magnitude of similarity as if they were at the same frequency. Typically, within a single instrument or a system of multiple instruments, frequency generation elements are phase-locked to a lower input frequency reference, so that a consistent phase relationship is expected to result. For example, an instrument or system of instruments may include digital signal processing structures with maximum clock rates in the hundreds of MHz, and a high-speed digital-to-analog converter (DAC) or analog-to-digital converter (ADC) with clock rates in the multiple GHz range. Phase coherence between the multiple channels of multi-channel instruments is often required, and tight synchronization of events within individual multi-channel instruments or across multi-channel systems is often essential and dependent on the phase coherence.
FIG. 1B illustrates an example of a reference frequency system in which multiple frequencies are derived from an input frequency f1 as a reference. In FIGs. herein, any frequency multiplier may be shown as a single block even though the frequency multiplier may itself be an electronic circuit that includes multiple components. The illustration of a frequency multiplier as a single block reflects that a frequency multiplier circuit is a circuit in which the frequency multiplier is functionally an individual component and the frequency multiplier circuit includes one or more additional components. In FIG. 1B, the reference frequency system 100 includes a first frequency multiplier 111a, a second frequency multiplier 111b, a third frequency multiplier 111c, and a fourth frequency multiplier 111d. In FIG. 1B, the input frequency f1 is analogous to the input frequency fin FIGS. 1A and 1s an input frequency reference that is input to the first frequency multiplier 111a, the second frequency multiplier 111b, the third frequency multiplier 111c, and the fourth frequency multiplier 111d. Overall, FIG. 1B illustrates a reference frequency system 100 in which multiple clocks are derived from a common clock. The derivation of multiple clocks as in FIG. 1B may be in the same instrument, or in multiple instruments with a common clock, or even at multiple sites. Each of the first frequency multiplier 111a, the second frequency multiplier 111b, the third frequency multiplier 111c, and the fourth frequency multiplier 111d may be in a different circuit, and each of these circuits may have different topologies. Each of the different circuits may also have their own phase-drift over temperature, so that the output signals from the different circuits have relative phase drifts (within bounds), even though the output frequencies of the output signals are all derived from the same common reference clock. Accordingly, minimizing the individual phase-drift of each circuit in FIG. 1B may be important regardless of the actual values of each derived frequency. Phase drift may occur even when two such circuits generate the same frequency, when, for example, the two frequencies are not co-located, or when the two circuits emphasize a different aspect of performance.
The first frequency multiplier 111a in FIG. 1B is a component of a first frequency translation circuit that also includes a first phase detector 120a, a first integrator 132a, and an oscillator 191. The first frequency multiplier 111a multiplies the input frequency f1 by N1 and outputs a first multiplied result f1×N1 to a first phase detector 120a. The first phase detector 120a is an example of a phase detector that detects a first phase difference between the first multiplied result f1×N1 and a second frequency f2. The first integrator 132a is an example of an integrator which provides a first integrated result to an oscillator 191. The second frequency multiplier 111b is a component of a second frequency translation circuit that also includes a digital-to-analog converter 196. The second frequency multiplier 111b multiplies the input frequency f1 by N2. The digital-to-analog converter 196 may be a high-speed digital-to-analog converter and may have direct digital synthesis. The third frequency multiplier 111c is a component of a third frequency translation circuit. The third frequency multiplier 111c multiplies the input frequency f1 by N3. The fourth frequency multiplier 111d is a component of a fourth frequency translation circuit that also includes a second phase detector 120b, a frequency divider 198, a second integrator 132b and a oscillator 192.
In FIG. 1B, the first frequency translation circuit outputs a first output signal at the second frequency f2. The second frequency translation circuit outputs a second output signal at the third frequency f3. The third frequency translation circuit outputs a third output signal at the fourth frequency f4. The fourth frequency translation circuit outputs a fourth output signal at the fifth frequency f5. The phases of the first frequency f1, the second frequency f2, the third frequency f3, the fourth frequency f4 and the fifth frequency f5 may drift (within bounds) relative to one another with time and temperature, even though the different frequencies are phase-locked together. The drift may occur even if any two or more of the first frequency f1, the second frequency f2, the third frequency f3, the fourth frequency f4 and the fifth frequency f5 is the same, such as when two translation circuits are not co-located or when they emphasize a different aspect of performance. The drift will impact the multi-channel coherence and synchronization of events within and across the reference frequency system 100, and this is typical for many multi-channel instruments. The drift can be an issue in a system where phase relation of signals at the input to the system and the output from the system are important.
Often, the primary contributor to drift for any particular multiplied result is the frequency multiplier that produces the multiplied result. A primary problem with frequency multipliers may lie with the filter(s) such as the filter 112. The filter(s) require sharp roll-off near the pass-band to sufficiently suppress the unwanted harmonics. Filters with sharp roll-off have a steep phase versus frequency slope well into the pass-band. When the filter shifts slightly due, for example, to temperature change, the phase delay through the filter will change at a given frequency. The phase of the output signal relative to the phase of the input signal becomes dependent on the temperature of the reference frequency system 100.
Frequency multipliers with shifting phases are therefore unstable and will benefit from stabilization as described herein.